This invention relates to electronic components and, in particular, to micro-electronic circuits which include soldered electrical leads.
Many types of electronic components utilize a soldered lead for electrical connection to the outside world. Such components include thin film, thick film, and hybrid integrated circuits where circuit elements are formed on an insulating substrate and leads are soldered to contact pads formed around the periphery of the substrate. The leads are usually formed from lead frames with the end of each individual lead shaped into a jaw configuration for clipping onto the contact pad. The leads usually include either a slug of solder positioned above the jaw or a solder cladding inside the jaw, which solder is reflowed during a heating step subsequent to the attachment of the leads. (See, e.g., Keller, "Significant Features of Solder Connections to Gold Plated Thin Films," IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. CHMT-5, No. 4, pp. 408-419, (December 1982).)
The leads are usually made of an alloy including copper, such as phosphor bronze which comprises approximately 95 percent copper and 5 percent tin. The solder is usually a mixture of Pb and Sn. It has been found that during aging, copper from the lead and tin from the solder form brittle intermetallic compounds on the lead surface which tend to weaken the bond and cause detachment of the leads. This effect is particularly important where the lead design does not include a clip member embedded within the solder to provide additional mechanical strength. The problem is also significant where the component must function at elevated temperatures for extended periods of time. In some applications, it is desirable that bonds withstand 100.degree. C. aging for at least 40 years.
It is therefore a primary object of the invention to provide electronic components which include strong, reliably bonded leads for external electrical connection.